Forming an ink pattern on the surface of a form cylinder of a printing press, which ink pattern exhibits, at least in part, a two-dimensional ink gradient extending in an axial direction and a circumferential direction on the surface of the form cylinder is known as such in the art. This principle was recently developed by Russian entity Goznak and is exploited in the context of so-called two-dimensional iris printing (hereinafter referred to as “2D-iris printing”). 2D-iris printing is in particular described in European patent application EP 1 053 887 and associated Russian patent RU 2 143 344 C1, as well as in Russian patent RU 2 143 342 C1.
An apparatus for carrying out 2D-iris printing is furthermore described in Russian patent RU 2 147 282 C1. FIG. 10 annexed hereto is an illustration of the apparatus disclosed in this document, which apparatus derives from the configuration of the multicolour offset printing press disclosed in Swiss patent CH 655 054 A5. Reference numeral 103 in FIG. 1 designates a plate cylinder carrying one offset printing plate, 102 designates a blanket cylinder carrying one blanket, 101 designates an impression cylinder, 104 designates an ink-collecting cylinder with two blankets, 105 designates four selective-inking cylinders (or chablon cylinders), and 106 designates four inking devices for inking the corresponding selective-inking cylinders 105 (which inking devices are only partially shown). In the configuration illustrated in FIG. 10, plate cylinder 103, blanket cylinder 102 and chablon cylinders 105 are each one segment cylinders, while impression cylinder 101 and ink-collecting cylinder 104 are two-segment cylinders (Swiss patent CH 655 054 A5 shows a similar machine configuration where the impression cylinder and the ink-collecting cylinder are three-segment cylinders). In other words, a ratio between the diameter of the chablon cylinders 105 and the diameter of the ink-collecting cylinder 104 is 1:2.
Each chablon cylinder 105 is inked by its associated inking device 106 and carries one chablon plate with raised portions corresponding to selected areas to be inked on the plate cylinder 103 in the desired colour. Each chablon cylinder 105 thus inks corresponding areas on each blanket of the ink-collecting cylinder 104 to form a multicolour ink pattern which is transferred onto the surface of the plate cylinder 103, thus inking the offset printing plate with a multicolour ink pattern. The resulting ink pattern corresponding to the printing form carried by the plate cylinder 103 is then transferred to the blanket cylinder 102, which in turn transfers the ink pattern onto the printed substrate which passes between the blanket cylinder 102 and the impression cylinder 101.
This inking principle whereby a same printing plate is inked with a multicolour ink pattern is also known under the designation of “Orlof” principle. It differs from the conventional multicolour inking principle used in conventional offset printing wherein a plurality of printing plates each corresponding to a desired colour to be printed are provided and wherein each printing plate is inked by only one associated inking device. With such conventional inking principle, and in contrast to the Orlof principle, the resulting ink patterns of the plurality of printing plates are collected or regrouped on a same blanket before being transferred onto the printed substrate. A major advantage of the Orlof principle resides in the fact that, as one plate is inked with a multicolour ink pattern, a perfect register between the different colours is guaranteed, which perfect register is more difficult to counterfeit, especially when the printed pattern is formed of fines lines, such as guilloche patterns. In contrast, according to the conventional inking principle, the register between the different colours will depend on the precision with which the various ink patterns of the printing plates are transferred and collected on the same blanket.
According to patent RU 2 147 282 C1, and as generally taught in European patent application EP 1 053 887, at least one of the chablon cylinders 105 is subjected to cyclic oscillation movements in both the axial direction and the circumferential direction. In other words, the chablon cylinder 105 oscillates both horizontally from left to right and vice versa, and is accelerated and decelerated with respect to a nominal rotational speed of the printing press. Accordingly, during each revolution of the oscillated chablon cylinder 105, a patch of ink is transferred onto the surface of the blanket cylinder 104 at a slightly offset position as compared to the patch of ink applied during the previous revolution. After a certain number of cylinder revolutions, there results an ink pattern on the surface of the blanket cylinder 104 and on the downstream-located plate cylinder 103 which exhibits at least in part an ink gradient extending in both the axial and circumferential directions.
According to patent RU 2 147 282 C1, the distribution of ink in the two-dimensions, i.e. along the axial direction and circumferential direction, is performed exclusively upon transfer of the ink from the oscillated chablon cylinder 105 to the ink-collecting cylinder 104. This implies that the distance over which the ink is distributed is determined exclusively by the oscillation amplitude of the chablon cylinder 105. Increasing the distance over which ink is distributed would therefore mean increasing the oscillation amplitude of the said cylinder, which is possible in practice only up to a certain extent. In the case of the solution described in the above-mentioned patent publications, the oscillation amplitude is for instance in the range of ±0.1 mm to ±2 mm (i.e. a total amplitude of between 0.2 to 4 mm).
Furthermore, according to RU 2 147 282 C1, the oscillated chablon cylinders 105 are one-segment cylinders having the same size as the plate cylinder 103, i.e. cylinders exhibiting a fixed diameter determined by the configuration of the machine and the printing length of the sheets to be printed. A typical diameter of the chablon cylinders 105 is for instance 280.20 mm (i.e. with a circumference of 880.274 mm), which diameter is adapted for the printing of sheets having a standard format of usually up to 700 mm×820 mm. According to the solution described in patent RU 2 147 282 C1, a two-segment ink collecting cylinder is further used, i.e. a cylinder having twice the size of the chablon cylinders 105. The solution of patent RU 2 147 282 C1 accordingly requires a substantial amount of space and is therefore difficult to install in a compact manner in the inking system of a printing press.
U.S. Pat. No. 2,733,656 discloses a multicolour printing press comprising a printing cylinder carrying a plurality of relief plates which are inked by a plurality of so-called preprinting rollers that are associated in pairs parallel to one another, each preprinting roller being thus brought into contact with the surface of the relief plates carried by the printing cylinder. This document is totally silent about the creation of any ink gradient, whether one-dimensional or two-dimensional, or any cylinder or roller arrangement for distributing the ink in an axial or circumferential direction and does not provide any means therefor.